CHARDON, Ohio — Wearing a T-shirt with “killer” scrawled across it, smirking and gesturing obscenely, a teenager was given three life sentences without parole this morning for opening fire last year in an Ohio high school cafeteria in a rampage that left three students dead and three others wounded. T.J. Lane, 18, had pleaded guilty last month to shooting at students in February 2012 at Chardon High School, east of Cleveland. Investigators have said he admitted to the shooting but said he didn’t know why he did it. Before the case went to adult court last year, a juvenile court judge ruled that Lane was mentally competent to stand trial despite evidence he suffers from hallucinations, psychosis and fantasies. Lane was defiant during the sentencing, smiling and smirking throughout, including while four relatives of victims spoke. He calmly unbuttoned his blue dress shirt to reveal the T-shirt reading “killer,” which the prosecutor noted was similar to one he wore during the shooting. Prosecutors say Lane took a .22-caliber pistol and a knife to the school and fired 10 shots at a group of students in the cafeteria. Daniel Parmertor and Demetrius Hewlin, both 16, and Russell King Jr., 17, were killed.